In the late 1950s, when digital computing machines were first becoming somewhat commonplace, it occurred to those working with computers that it would be desirable to construct an arrangement whereby computers could communicate with each other and whereby operators at a remote operator location could communicate with a distant computer. While it would have been possible to construct a new communications network, it was apparent at the time that a far reaching pre-existing communications system, the telephone network, already existed. Since the telephone network was so far reaching, and already in place, the problem presented was how to allow the computers and remote devices (either other computers or terminals) to communicate over a channel designed to carry the human voice.
The solution to this problem was a device known as a data communications adapter, commonly referred to as a "modem"; modem being a contraction of "modulator-demodulator". The basic solution arrived at has become so widely used and popular that it is still the most common scheme for telephone network communication between computing devices. Basically this solution is the modulate data being transmitted through a frequency shift keyed (FSK) modulation scheme in a frequency band appropriate for transmission over voice grade telephone lines. The FSK signal is demodulated at the receiving end.
According to the most common North American standard for modems, two modes of operation are defined, called originate and answer, each mode having a pair of frequencies called mark and space defined for each mode. Thus, full duplex communication is possible wherein each of a pair of modems receives on one frequency pair and transmits on the other simultaneously.
Some more recent modems have increased the information density transmitted over voice grade telephone lines by using phase modulation schemes. However, it will be recognized by those skilled in the art that both phase modulation and frequency shift keyed modulation are all species of angle modulation and that generically angle modulation is used in most all modern modems for communication over voice grade telephone lines.
More recently, intelligent modems have been devised wherein decision making capability is built into the modem per se and the modem will perform functions in addition to the modulation and demodulation of data. For instance, modern intelligent modems will accept coded signals and provide and respond to common supervisory signals on the telephone line. For example, modern intelligent smart modems will, in response to commands; go off-hook, wait for dial tones, perform the dialing function, and answer incoming calls.
In the past, the number of components used to construct a modem tended to increase more or less linearly with the number of functions the modem performed. The provision of separate ring detection circuitry for a modem arranged to answer a telephone is an example.
Implementation of the demodulation circuitry was originally rather expensive but has, within the last decade, become less expensive due to the availability of phase lock loop integrated circuits, the phase detector output of which is commonly used to provide demodulated data at the receiving end.
Still more recently, modem functions have been fabricated on monolithic integrated circuits. However, these integrated circuits require limited input signals of a very precise 50% duty cycle to successfully demodulate incoming data. Thus, the symmetry of a squarewave derived from the incoming signal is critical and the complexity of circuitry preceding the input to the demodulator accordingly increases.